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Filed May 15, 1937 2 Sheets-Sheet 1 FIG! LSCHOTT sawmwr A TTORNEY UNITED STATES PATENT ()FFICE TRANSMISSION CIRCUIT Lionel Schott, East Orange, and. Sumner B. Wright, South Orange, N. J assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application May 15, 1937, Serial No. 142,832

6 Claims.

This invention relates to a transmission circuit and particularly to a transmission circuit which includes voice-operated devices.

An object of the invention is to improve the operation of voice-operated devices associated with transmission circuits.

A more specific object of theinvention is to automatically adjust the characteristics of voiceoperated devices in accordance with certain characteristics of the signal waves applied thereto in order to obtain optimum performance of said devices.

It is common practice, of course, to associate with certain types of speech transmission circuits, voice-operated devices, such as echo suppressors, amplifier-detectors, vogads (gain control device) compandors and the like. It has appeared to applicants that the operation of such devices could in many cases be improved if means were provided for indicating which particular range of frequency of the transmitted band is carrying the greatest power. Having such an indication, that is, that one particular band in the frequency spectrum of waves being transmitted is most prominent, certain adjustments of and connections to the voiceoperated device could be made whereby that prominence would be taken advantage of. For

example, that particular one of a plurality of input circuits associated with the voice-operated device having most suitable characteristics for the indicated range might be selected and connected to the voice-operated device. v

In accordance with a feature of the invention the band of frequencies being transmitted is divided into a plurality of smaller bands and indication is then made of that particular smaller band in which the power is greatest.

In accordance with a further feature of the invention indications are made, not only of the most prominent band, but also of the order of prominence of all the bands, that is for example maximum power, intermediate power and minimum power.

In accordance with a specific embodiment of the invention three band-pass filters are provided being effective to divide the speech frequency band being transmitted into three smaller bands. The power in each of these three bands is then measured by means which includes a series of relays so poled that the current from the most prominent band, with respect to the power therein, will cause operation of all the relays directly associated with that band and will oppose operation of the relays directly associated with the other bands.

A complete understanding of the arrangements contemplated by the invention and of the various advantageous features thereof will be gained from consideration of the following detailed description and the accompanying drawings in which:

Fig. 1 illustrates a circuit embodying features contemplated by the invention for indicating in which of three frequency bands the power is greatest;

Fig. 2 illustrates a circuit similar to that of Fig. 1 in which means have been provided to indicate the order of prominence of the three frequency bands with respect to the power therein; and

Fig. 3 illustrates schematically one application of the arrangement illustrated in Fig. 1 in connection with the automatic adjustment of certain I characteristics of voice-operated devices.

Referring now to Fig. 1, three band-pass filters 2|, 22 and 23 are shown connected in parallel across line 26. These filters serve to divide the frequency range received from line 26 into three smaller bands, it being assumed that filter 2! passes the high band, filter 22 the medium" band and filter 23 the low" band. (Band-pass filters are widely used in transmission circuits and their design and method of operation are described in a number of patents such as, for ex-- ample, G. A. Campbell Patent 1,227,113, issued May 22, 1917, Electric wave-filter.")

-A rectifier is associated with each of the three filters (rectifier 21 being associated with filter 2|, rectifier 28 being associated with filter 22 and rectifier 3| being associated with filter 23) which rectifiers serve to change the alternating current output therefrom into direct current. Condensers 32, 33 and 3B are provided to smooth the current impulses of the rectified waves in order that differential relays 4|, 42, 43, 46, 4'! and 48 will respond more to the general wave shape of the speech than to the instantaneous values of the current. The small arrows shown on the differential relays referred to indicate by their direction the effect of current passing through the windings. For example, referring to relay 4 I, current passing through the left-hand winding tends to operate the relay, that is, to move the armature to the left or away from the contact while current in the right-hand winding tends to oppose operation of the relay, that is to hold the armature in engagement with the contact as shown. Casual inspection will reveal that the relays are so arranged that current from each filter passes through one winding of each of four relays and that in each instance the current in two relays tends to operate these relays and in the other two relays tends to oppose operation.

To the right of the broken line, three relays, 52 and 53, are shown, one relay being associated with each pair of the differential relays; that is relay 5| is associated with relays 4| and 42, relay 52 is associated with relays 43 and 46 and relay 53 is associated with relays 41 and 48. These three relays are merely illustrative of one arrangement which may be utilized to provide an indication of the particular range in which the power is greatest. A lamp, or other type of indicator, may, if desired, be controlled by these relays or, in fact, may be directly substituted for the relays. Further, While the circuit arrangement disclosed is such that these relays are normally in nonoperated position when speech is not being applied to the circuit, the arrangement may, if.

desired, be such that the relays will remain in the condition last obtaining when speech was applied.

Resistance 56 and battery 51 are associated. with the line leading to relay 5|, resistance 58 and battery 6| are associated with the line leading to relay 52 and resistance 62 and battery 63 are associated with the line leading to relay' 52 and resistance 62 and battery 63 are associated with the line leading to relay 53. Relays 5|, 52 and 53 are normally shorted by paths leading respec--. tively to grounds 66 and 61, to grounds 68 and 1| and to grounds 12 and 13.

In order to further describe the arrangement of Fig. 1, let us suppose that the range of frequencies being passed by filter 2| is the most powerful and let us see how operation of relay 5| is brought about to indicate this fact, relays 52 and 53 at the same time to remain unoperated. It may be further assumed that the medium power is existent in the range passed by filter 22 and that the lowest power-is in the range passed by filter 23.

The current passed by filter 2| after rectification by rectifier 21 passes through the left winding (aiding) of relay 4|, right winding (opposing) of, relay 43, right winding (opposing) of relay 48, left winding (aiding) of relay 42 and back to the other side of line 26. Now as the direction of the current through relays 4| and 42 is in the aiding direction these two relays will operate while as the direction of the current through relays 43 and 48 is in the opposing direction, the latter two relays do not operate.

Operation of relays 4| and 42 interrupts the short-circuit path of relay 5| to respective grounds 66 and 61 and establishes an efiective operating circuit for relay 5| traced from battery 51, resistance 56, winding of relay 5| to ground 16. Relay 5| now operates as an'indication that the power in the range passed by filter 2| is greatest.

Now let us see the effect on the circuit of the medium power and the low power currents passed respectively by filters 22 and 23. The medium powered current passed by filter 22, after being rectified by rectifier 28, passes through the left winding (aiding) of relay 43, right winding (opposing) of relay 41, right winding (opposing) of relay 42, left winding (aiding) of relay 46 and back to the other side of line 26. Now while the,

hand winding. Therefore, while the short-circuiting path of relay 52 to ground 68 is interrupted, the short-circuiting path to ground 1| remains and an effective operating circuit for relay 52 is not set up. The opposing efiect of the medium powered current passing through the right winding of relay 42 does not, of course, prevent operation of this relay by the more powerful current passing through the left winding in the aiding direction.

Considering now the effect of the low powered current passed by filter 23, this current after rectification by rectifier 3| passes'through the left winding (aiding) of relay 41, right winding (opposing) of relay 46, right winding (opposing) of relay 4|, left winding (aiding) of relay 48 and back to the other side of line 26. None of the relays is operated by this current, the aiding efiect in relays 41 and 48 being ofiset by the greater opposing effect of the medium power current and the high power current respectively. Neither of the short-circuiting paths of relay 53 is interrupted therefore and the relay remains unoperated. The opposing efiect of the lowpower current on relays 46 and 4| does not, of course, prevent operation of these relays by the 7 more powerful currents which pass through the power be existent in the current passed by filter 212, relay 52 will be operated and should maximum power be existent in the' current passed by filter 23 relay 53 will be operated.

Referring now to Fig. 2 a modification of the arrangement described above is illustrated in accordance with which indication is given not only of the most prominent band with respect to power but. also the order of prominence of all the bands, i. e., which band is most powerful, which band is second in power and which is third in power. Here again as in the instance of the circuit illustrated in Fig. 1, three band-pass filters 8|, 82 and 83 are provided which are efiective to divide the frequency band being transmitted over line 86 into three. smaller bands, it being assumed that filter 8| passes the high band, filter 82 the medium band and filter 83 the low band. As in the circuit of Fig. 1, the output of each filter is connected through one winding of each of four relays, relays 81, 88, SI, 92, 93 and 95 of the present circuit corresponding in general, as to arrangement and operation, to relays 4|, 42, 43, 46, 41 and 48 of Fig. 1. Three lamps with relays for controlling the energization thereof have, in accordance with the present modification, been associated with each filter channel, lamps H2 and H3 and relays H6, H1 and H8 being associated with filter 8|, lamps I2I, I22 and I23 and relays I26, I21 and I28 being associated with filter 82 and lamps I3I, I32 and I33 and relays I36, I31 and I38 being associated with filter 83.

- Each of the three lamps of a group is preferably marked in a distinctive manner in accordance I with the order of prominence of power which it amuis intended to indicate when energized, for example, lamp III may be marked maximum, lamp II2 may be marked "intermediate and lamp II3 may be marked minimum" and so on.

contact of relay H8 to ground I57. Lamp III therefore lights to indicate that the maximum powered band is being passed by filter 8 I. Lamp II2 cannot light as its energizing path is open 5 Let us assume now for purposes of further deat-the break contact of relay I I8 (operated) while 5 scription that maximum power is present in the lamp II3 likewise cannot light as its energizing range of frequencies passed by filter 8I, that the path is open at the make contact of relay II6 next to maximum or the intermediate power is (unoperated). present in the range of frequencies passed by fil- Let us see now what happens in the group of ter 82 and that the lowest or minimum power is lamps (lamps I2I, I22 and I23) associated with fil- 10 present in the range of frequencies passed by ter 82. As this filter is passing the medium powfilter 83. To properly indicate these conditions ered range,lamp I22 should be lighted as this lamp the maximum" lamp associated with filter 8I, is the intermediate power indicator. It will i. e., lamp III, should be lighted, the interbe recalled that relay 92 is operated while relay mediate lamp associated with filter 82, i. e., 9I remains unoperated, i. e., in the position illus- 15 lamp I22 should be lighted and the minimum trated. We have, therefore, a path traced from lamp associated with filter 83, i. e., lamp I33, battery I6I, through the windings of relays I28 should be lighted. Let us trace the operation and I27, make contact of relay 92, resistance I62 of the circuit in order to determine how such to ground I63. Here we do not have, as in the multiple indication is accomplished. instance of the operating circuit of relays II7 20 The operation of relays 87, 88, SI, 92, 93 and and II8, parallel resistance paths to ground but 96 is the same as that of the corresponding reonly a single resistance path. The current which lays of Fig. 1 described in detail above and will flows in this path is not as great as in the instance be described only briefly, therefore. The maxiof the parallel resistance path traced above and,

25 mum powered current passed by filter 8|, after while sufficient to operate relay I27, is not suffi- 25 being rectified by rectifier I 4|, passes through the cient to operate relay I28 which is designed to left winding (aiding) of relay 87, right winding have a higher operate value than that of relay (opposing) of relay 9 I, right winding (opposing) I27. Likewise the current which flows in the path of relay 96 and left winding (aiding) of relay 88, traced from battery I66, winding of relay I26,

relays 87 and 88 operating and relays SI and 96 break contact of relay 9| (unoperated), resist- 30 being held in unoperated position by this current. ance I67 to ground I68 is not sufficient to hold The intermediate powered current passed by relay I26 in operated position and the relay therefilter 82 after being rectified by rectifier I42 fore drops back to unoperated position. passes through the left winding (aiding) of relay Operation of relay I27 establishes an energiz- SI, right winding (opposing) of relay 93, right ing path for lamp I22. traced from battery I66, 35 winding (opposing) of relay 88 and left winding make contact of relay I27,lamp I22,break contact (aiding) of relay 92. Only relay 92 is operated of relay I28 (unoperated) to ground I7I. Lamp by the intermediate powered current, the oppos- I22 therefore lights to indicate that the intering efiect of the more powerful current in the mediate power current is being passed by filter right winding of relay 9I preventing operation 82. Lamp I2I cannot light as its energizing cir- 40 thereof. cult is open at the make contact of relay I28 The low powered current passed by filter 83 while lamp I23 cannot light as its energizing after being rectified by rectifier I43, passes path is open at the make contact of relay I26. through the left winding (aiding) of relay 93, Referring now to the group of lamps associated right winding (opposing) of relay 92, right windwith filter 83 (lamps I3I, I32 and I33) we should 45 ing (opposing) of relay 87 and left winding (aidexpect to find lamp I33 operated, this lamp being ing) of relay 96. None of the relays is operated the minimum power" indicator. It will be reby the low powered current, however, as in the membered that neither relay 83 nor relay 96 is instance of relays 83 and 86, the more powerful operated. We have therefore an energizing path current in the opposing direction prevents operafor relay I36 traced from battery I76, winding of 50 tion thereof. relay I36 through two parallel resistance paths As a result of the currents passed by the three to ground, one path from break contact of relay filters therefore, relays 87, 88 and 92 are oper- 96 through resistance I77 to ground I78 and the ated while relays SI, '93 and 96 are held in the other path through break contact of relay 93 unoperated position illustrated. through resistance I8I to ground I82. Suflicient 55 Operation of relays 87 and 88 completes a path current will flow from battery I76 over these traced from battery I46, through the windings of parallel resistance paths to ground to cause operrelays H8 and II7 to ground through two paration of relay I36 thereby providing an energizallel resistance paths, one. path through make ing path for lamp I33 traced from battery I83,

contact of relay 88, resistance I47 to ground I48 lamp I33, make contact of relay I36 to ground 60 and the other path through make contact of re- I86. Lamp I33 lights therefore to indicate that lay 87, resistance I5I to ground I52. In view the minimum power current is being passed of the parallel connection of resistances I47 and by filter 83. Lamp I3I cannot light as the ener- I5I in the path the current flow is suflicient to gizing path thereof is open at the make contact cause operation of relays II7 and H8. Operaof relay I38 and lamp I32 cannot light as the 65 tionof relays 87 and 88 also interrupts the enenergizing path thereof is open at the make ergizing path for relay I I6 (battery I53, winding contact of relay I37, of relay H6, and the break contacts of relay It will be understood that the circuit will oper- 87 and 88 through resistance I47 to ground I48 ate in a similar manner for other distributions and through resistance I5I to ground I52 respecpf power, for example, should the maximum 70 tively) and relay I I6 therefore drops back to unpower current be passed by filter 82, the medium operated position. power current by filter 83 and the minimum power Operation of relay II8 establishes an enercurrent by filter 8I, lamps I2I, I32 and H3 gizing path for lamp III. (maximum power inwould be lighted.

dicator) traced from battery I56, lamp III, make Referring now to Fig. 3, a practical applica- 75 tion of the power analyzer circuit of Fig. 1 is illustrated. There is shown in this figure, in partial schematic, a two-way four wire terminal of a radio telephone system. The four wire line comprises a transmitting channel 2|! and a receiving channel 2!2. Transmitting channel 2H includes a compressor 2 I 3 while receiving channel 2! 2 includes an expander 2!6, the two (compressor and expander) constituting a compandor of the general nature disclosed in Doha Patent 2,018,489 issued October 22, 1935. The application of a compandor to a terminal circuit of the general nature illustrated is described in an article The Compandor-An Aid Against Static in Radio Telephony, by R. C. Mathes and S. B. Wright, which was published in Electrical Engie neering of June 1934, vol. 53, pages 860 to 866.

The output from subscribers station 2!! is coupled to transmitting channel 2!! through windings of hybrid coil 2I8. Potentiometer 23! may be utilized to regulate the volume of this output which is then passed to amplifier 232 by transformer 233. The output of amplifier 232 is connected through relay circuits 236 to amplifier 237. The output of amplifier 23! is connected through transformers 238 and 25! to compressor 2!3. The portion of the circuit described in this paragraph is similar to that fully disclosed in Patent 2,084,457 issued June 22, 1937,

to L. Schott, entitled Transmission systems.

Compressor 2!3 is of' the type disclosed in Doha Patent 2,018,489 referred to above; three digit numbers in the 400 series used herein areintended to correspond to two digit numbers used in the Doha patent, the last two digits of the three digit numbers of the 400 series used herein being the same as the numbers of the Doha patent with which they are. intended tocorrespond. For example, the vacuum tube of Fig. 3, designated 4, is the same as tube !4 of the Doha patent, the vacuum tube designated 440 of Fig. 3 is the same as tube 40 of the Doha substation 2!! and passed totransmitting chanpatent, battery 428 of Fig. 3 is the same as battery 28 of the Doha patent, and so on.

The output of compressor H3 is coupled to radio transmitter252 through transformer 253. Receiving channel 212 is shown in a more or less schematic manner, the output of radio receiver 256 being coupled to the input of subscribers set 2!! through windings of hybrid coil 2!8, automatic volume control device 251, ex pander 2l6 (as disclosed in Doha Patent 2,018- 489) and amplifier 258' being included in the channel.

Speech power analyzer 21!, which preferably is of the type illustrated in Fig. l and described above, is connected across transmitting channel 2! at the constant volume point beyond repeater 232, by line 2'12. A bridging pad 273 together with amplifier 214, the gain of which is suflicient to supply the necessary output power, are included in line 212.

Transmitting amplifier-detector 216 is also bridged across transmitting channel 2! I, at the constant volume point, by line 211. I

Heretofore it has been the usual practice in the instance of such voice operated devices as amplifier-detector 216 to tune the input transformer circuit associated therewith rather broadly by means of a series condenser and a resistance, thereby producing a circuit effective at the more important voice frequencies but ineiiicient at other frequencies. Ithas appeared to applicants, however, that the operation of such devices will be improvedif a plurality of somewhat narrowly tuned circuits be provided and if the input circuit associated with the amplifier-detector at any one time be selected in some accordance with an indication of which band of frequencies is the highest powered. There is disclosed, therefore, in association with amplifier-detector 2'16 and line 217, a plurality of tuned circuits 29!, 292, 293 and 296, each of which is tuned to operate most efficiently with a difierent band of frequencies. Through operation of speechpower analyzer 21! that particular input circuit which is tuned to the particular band of frequencies in which the power is maximum at a particular instant is selected and associated at that instant with the input transformer of amplifier-detector 216. The exact circuit operations involved will be described subsequently.

Heretoforeit has been the usual practice in the instance of such devices as compressor 2l3 to connect a condenser of predetermined magnitude across the input to the rectifier circuit (i. e., referring to Fig. 3 across the input to vacuum tube 443 just to the left of resistance 446) in order to provide a time constant so that the voltage across the resistance will vary according to the syllable frequency. It has appeared to applicants that here again the operation would be improved if, instead of a single condenser, several condenser circuits of difierent time constants were to be provided and selected for association with the compressor circuit in coordi-' nation with the selection of the tuned input circuits referred to above. Accordingly a plurality of condensers 3!!, 3l2, 3!3 and 31B, of'diiferent time response characteristics, are provided; the circuit operations involved in their selection and association with the compressor circuit in coordination with selection and association of the tuned input circuits with the amplifier-detector will be described subsequently.

A part of the voice currents originating at nel 2| through windings of hybrid coil 218, pass through the amplifiers, the delay circuits and the compressor included in the transmitting channel and are transmitted by radio transmitter 252. A part of these voice currents is shunted over line 212 to speech analyzer 21! which is of the tion of relay 33! completes an obvious operating circuit for relays 332 and 333.

Relay 332' by its operation removes the short circuit previously applied through its break contact to transmitting channel 2! thereby enabling" the channel for passage of voice currents to radio transmitter 252.

Relay 333 by its operation completes a short-circuiting path across receiving channel 2l2, thereby disabling the receiving channel.

A relay is associated with each of the tuned input circuits, relay 336 being associated with input circuit 29!, relay 33'! with input circuit 292,

relay 338' with input circuit 293 and relay 35! with input circuit 296. Input circuit 29! is what may be termed the normal input circuit, being connected to the input of amplifier-detector 215 at all times when none of the other three input circuits is so connected. It will be assumed that input circuit 292 is tuned to the band of frequencies passed by the high filter of the analyzer circuit, that input circuit 293 is tuned to the band of frequencies passed by the medium filter and that input circuit 298 is tuned to the band of frequencies passed by the "low filter.

It will be assumed, further, that condenser 3H furnishes the best time response for the high frequency band, condenser 3l2 the best time response for the medium frequency band, and condenser 3| 3 the best time response for the low" frequency band. Condenser 3I6 furnishes the normal time response and is connected across the input of the compressor rectifier at all times when none of the other condensers are so connected.

Let us assume for purposes of further description that th speech shunted over line 212 is analyzed by speech power analyzer 21l, in the manner described above, and that it be found that the current passed by the high filter (corresponding to filter 2i of.Flg. l) is the most powerful. As previously stated we are assuming that input circuit 292 and condenser 3| l are most suitable for use with this particular frequency.

The indicator relay associated with the high filter of analyzer 211 (corresponding to relay 5! of Fig. 1) will operate and let us assume that by so doing contacts are made to connect battery to conductor 352. We have, therefore, a path from battery over conductor 352, winding of relay 331 to ground 353, as well as a path from battery over conductor 352, winding of relay 356 to ground 351, relays 331 and 356 operating over these paths.

Operation of relay 331 connects (through-its make contacts) tuned circuit 292 to the input of amplifier-detector 216 and also interrupts (at its break contact) the energizing path of relay 336 (battery 358, break contacts of relays 351, 338 and 331, winding of relay 336 to ground 31!),

whereupon relay 336 drops back to unoperated position and disconnects "normal input circuit 291 from amplifier-detector 216.

Operation of relay 35B connects (through its make contact) condenser 3| l across the input of the compressor rectifier in parallel to resistance 446 and also interrupts (at its break contact) the energizing path of relay 312 (battery 313, break contacts of relays 356, 316, and 311, winding of relay 312 to ground 318) relay 312 thereupon dropping back to unoperated position and disconnecting normal condenser 3|6 from the input of the compressor rectifier. It will be apparent that the proper tuned input circuit (circuit 292) and the proper time response element (condenser 3H) for the band of frequencies in which the power is greatest are selected and associated with the amplifierdetector and the compressor respectively.

It will be understood that the arrangement functions in a similar manner for other current distributions. For example, had maximum power been existent in the band passed by the medium" frequency band filter of speech analyzer 211 (corresponding to filter 22 of Fig. 1), battery would have been connected to lead 39l and relays 338 and 316 would have operated thereby associating input circuit 293 with amplifier-detector 216 and condenser 312 with the compressor rectifier. Again had maximum power been existent in. the frequency band passed by the low filter of speech analyzer 211 (corresponding to filter 23 of Fig. 1) battery would have been connected to lead 392 and relays 35| and 311 would have operated connecting tuned circuit 296 to amplifier-detector 216 and condenser 3I3 to the input of the compressor rectifier. During such times as battery is not applied to leads 352, 39l or 392, normal" input circuit 29! and normal condenser 3l6 are associated with the amplifierdetector and the compressor rectifier respectively.

Reception of speech waves over receiving channel 2l2 is in accordance with well understood and previously published methods and need not be described herein. However, it may be pointed out that a part of the received speech waves is passed over line 393 to receiving amplifier-detector 394. Operation of the amplifierdetector by these speech waves results in turn in operation of relays 395, 396 and 391.

Relays 395 when operated acts to increase the gain of automatic volume control device 251 in a manner described in detail in Patent 2,084,457 issued June 22, 1937, to L. Schott entitled "Transmission systems. Relay 396, when operated, interrupts the output circuit of transmitting amplifier-detector 219. Relay 391 when operated completes a short circuiting path across line 212 which is thereby "disabled for transmission to speech power analyzer 21L This assures that the speech power analyzer will be operated by transmitted speech only and not by received speech. It will be obvious, however, that a separate speech power analyzer may be provided in the receiving channel if desired, operating in a manner similar to that described above for selecting the proper tuned input circuit for association with receiving amplifier-detector 394 and the proper time response element for association with expander 2|6.

While the arrangement described above and illustrated in Fig. 3 is directed to the specific idea of determining which band is passing the maximum power and then making suitable adjustment in accordance with such determination, it is contemplated also that it may be desirable in certain cases to obtain an indication of that particular band which is passing the greatest power relative to its normal or average transmitted power. By way of example, let us assume that the average power of the band passed by the high filter is .2 milliwatt that the average power of the band passed by the "medium filter is l milliwatt and that the average power of the band passed by the "low filter is .5 milliwatt. Now if the speech wave energy should change for any reason, for example due to a change of the talkers, the power distribution might then be such that the power in the band passed by the high filter would be .1 milliwatt, the power in the band passed by the medium" filter .8 milliwatt, and the power passed by the low filter would be .7 milliwatt. The significant change for which it might be desirable to set up special adjustment in the voiceoperated device would be the increase of .2 milliwatt above normal in the band passed by the "low filter even though the band passed by the medium filter would still be of maximum power. The circuit illustrated in Fig. 1 may be modified to provide such an indication merely by so preadjusting the sensitivities of the three rectifiers, for example by the use of properly designed attenuators, that the outputs from all three will be the same for the average power distributions first mentioned. That is, we would add a certain resistance in the medium band circuit and somewhat less resistance in the low" band circuit, bringing the output of each down to equal Room that from the high band circuit, where we have the lowest average initial power. Now it is obvious that, having preadjusted forequal outputs with the initial average power distributions, we

will receive a maximum power indication for that particular band in which the greatest increase above this initial average power distribu-' tion occurs.

While certain specific embodiments of the invention have been selected for illustration and detailed description, the invention is not, of course, limited in its applicationto those embodiments. For example, while means are described for dividing the speech band to be analyzed into three smaller hands it may, of course, be divided into two bands or into more than three bands. Further, other applications of the speech analyzer than those described are, of course, contemplated. The embodiments described and illustrated should be considered as descriptive ratherthan restrictive.

What is claimed is:

1. In combination, a transmission line, means for applying waves of a comparatively wide frequency range to said line, means for splitting said waves into a plurality of smaller frequency bands, the number of said bands being greater than two, means for measuring the power passed in each of said smaller frequency bands relative to that in the other bands, and means for indicating the order of prominence among said smaller bands with respect to the power passed therein.

2. In combination, a transmission line, means for applying waves of a comparatively wide frequency range to said line, a plurality of band-pass filters for dividing said waves into a plurality of groups of waves of smaller frequency bands, the number of said groups being greater than two, means for measuring the power passed in each of said smaller bands relative to that in the other bands, and means for indicating the order of prominence among said. smaller bands .with respect to the power passed therein.

3. In combination, a transmission line, means for applying waves of a comparatively wide frequency range to said line, means for splitting, said waves into a plurality of groups of smaller frequency bands, the number of said groups being 4. In combination, a transmission line, means for applying-waves of a comparatively wide frequency range to said line, means for splitting said waves into a plurality of smaller frequency bands, and means for measuring the power passed in each of said smaller bands relative to that in the other bands, said last-mentioned means including a series of relays so poled and connected I that current in each of said smaller bands will tend to aid operation of a plurality of said relays and will tend to oppose operation of an equal number of said relays.

5. In combination, a transmission line, means for applying waves of a comparatively wide frequency range to said line, a plurality of channels connected to said line, frequency discriminative means for dividing said waves into a plurality of groups of waves of smaller frequency bands, the number of said bands being greater than two, each of said channels passing one of said smaller bands, a maximum indicator, an intermediate indicator and a minimum indicator associated with each of said channels, and means for measuring the power passed in, each of said smaller bands and for causing the operation of one indicator associated with each channel in order to indicate the order of prominence among said smaller bands with respect to the power passed therein.

6. In combination, a transmission line, means for applying waves of a comparatively wide frequency range to said line, three channels connected to said line, a plurality of filters for dividing said waves into a plurality of -groups of waves of smaller frequency bands, one of said filters being associated with each of said channels, each of said channels passing one of said smaller bands, a relative energy indicator associated with each of said channels, two relays associated with each channel, operation of both relays being effective to cause operation of the indicator associated with that respective channel, and means whereby the current passed by each of said filters is passed through a winding of thetwo relays associated with that respective channel and through a winding of one relay associated with each of the other two channels, said current being passed through LIONEL SCHOTI. SUIVINER B. WRIGHT. 

